Digital signature collection and authentication

ABSTRACT

A digital signature collection and authentication system includes an ink pen having an ultrasonic transmitter that transmits ultrasonic energy to a plurality of ultrasonic receivers. A computer triangulates the location of the pen versus time to generate the signature shape, and to generate velocity and acceleration data. The pen also includes a pressure sensitive tip to record pressure applied to the pen tip. The pen also includes a higher frequency burst transmitter useful to generate a time reference, and to transmit the pressure information. The computer packetizes the shape, velocity, acceleration, and pressure data with a time stamp and an IP address or phone number, encrypts the packet and sends it to a host computer for authentication.

FIELD

[0001] The present invention relates generally to data authentication,and more specifically to systems for digital signature collection andauthentication.

BACKGROUND

[0002] Digital signature authentication is becoming more important asmore people conduct business electronically. For example, whenelectronically banking, electronically filing taxes, and when enteringcontracts over the internet, a digital signature may be collected andauthenticated.

[0003] Researchers and vendors have been working to find ways toauthenticate digital signatures, and to verify that those signatureshave been validly collected. Signatures can be collected using pentablets such as those commercially available from Interlink Electronicsof Camarillo, Calif., and Wacom Co. Ltd., of Vancouver Wash. These pentablets, and other similar devices, collect a signature shape forrecording by using a pressure sensitive tablet. Some pens are tethered,and some are not. Typically, the tablet is connected to a port on acomputer, and the shape of the signature is sent from the tablet to thecomputer through the port.

[0004] Authentication is typically limited to the data that is collectedat the time of the signature. For example, signature shapes collectedusing pen tablets are typically compared against known shapes todetermine authenticity. The above techniques and productsnotwithstanding, digital signatures are still fairly easily forged, inpart because the shape of the signature is often the only criteriacollected, and therefore the only criteria used for authentication.

[0005] For the reasons stated above, and for other reasons stated belowwhich will become apparent to those skilled in the art upon reading andunderstanding the present specification, there is a need in the art foran alternate methods and apparatus for digital signature collection andauthentication.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006]FIG. 1 shows a signature collection and authentication system;

[0007]FIG. 2 shows a signature collection system and remoteauthentication system;

[0008]FIG. 3 shows an ultrasonic pen;

[0009]FIG. 4 shows a block diagram of a first ultrasonic pen;

[0010]FIG. 5 shows a block diagram of a second ultrasonic pen;

[0011]FIG. 6 shows a block diagram of receivers for use in conjunctionwith a signature collection system; and

[0012]FIG. 7 shows a flowchart of a method for collecting signatures tobe authenticated.

DESCRIPTION OF EMBODIMENTS

[0013] In the following detailed description of the embodiments,reference is made to the accompanying drawings which show, by way ofillustration, specific embodiments in which the invention may bepracticed. In the drawings, like numerals describe substantially similarcomponents throughout the several views. These embodiments are describedin sufficient detail to enable those skilled in the art to practice theinvention. Other embodiments may be utilized and structural, logical,and electrical changes may be made without departing from the scope ofthe present invention. Moreover, it is to be understood that the variousembodiments of the invention, although different, are not necessarilymutually exclusive. For example, a particular feature, structure, orcharacteristic described in one embodiment may be included within otherembodiments. The following detailed description is, therefore, not to betaken in a limiting sense, and the scope of the present invention isdefined only by the appended claims, along with the full scope ofequivalents to which such claims are entitled.

[0014] The method and apparatus of the present invention provide amechanism to collect and authenticate digital signatures. A pen with anultrasonic transmitter transmits ultrasonic pulses to a plurality ofreceivers. The receivers provide time-of-arrival information to acomputer that triangulates the position of the ultrasonic pen versustime. The location information is stored as the signature shape.Velocity and acceleration of the ultrasonic pen are calculated andbundled with the signature shape as part of the collected signature tobe authenticated. The pen has a pressure sensitive tip to collect datarepresenting pressure applied to the tip of the pen. The pen alsoincludes a higher frequency burst transmitter that provides a timereference, and also transmits the pressure information. The pressureinformation is also bundled with the signature shape as part of thecollected signature to be authenticated. In some embodiments, furtherdata is bundled as part of the collected signature to be authenticated.For example, a time stamp and an address of the computer can be bundled.The computer then makes a comparison between the collected signature anda known signature, or the computer can send the collected signature to adifferent computer for the comparison.

[0015]FIG. 1 shows a signature collection and authentication system.System 100 includes computer 110 with storage medium 180, receivers 120and 130, and ultrasonic pen 140. Ultrasonic pen 140, as is explained inmore detail below, includes an ultrasonic transmitter used totriangulate the position of the pen. In some embodiments, ultrasonic pen140 also includes an ink cartridge capable of writing on a surface suchas paper. As show in FIG. 1, signature shape 160 is written in ink onpaper 150.

[0016] Ultrasonic pen 140 emits ultrasonic energy in pulses. In someembodiments, pulses are emitted at a rate of between 50 and 100 pulsesper second. In some embodiments, ultrasonic pen 140 includes a higherfrequency transmitter in addition to an ultrasonic transmitter. For thepurposes of this description, a “higher frequency” is any frequencyhigher than that used for ultrasonic communication. Examples of higherfrequencies include frequencies in the infrared (IR) spectrum, and radiofrequency (RF) spectrum. In embodiments that include a higher frequencytransmitter in ultrasonic pen 140, a corresponding higher frequencyreceiver is included in system 100. In the embodiments represented byFIG. 1, receiver 120 includes antenna 170 to receive higher frequencybursts from ultrasonic pen 140. The higher frequency transmitter is alsoa burst transmitter that can be used as a time reference for receivers120 and 130. Because the velocity of the higher frequency signals ismuch greater than the velocity of the ultrasonic energy, the higherfrequency bursts are used as a time reference.

[0017] Receivers 120 and 130 are ultrasonic receivers that receiveultrasonic bursts from ultrasonic pen 140, and provide time-of-arrivalinformation to computer 110. Receivers 120 and 130 are located on asurface (not shown) preferably in the same plane as paper 150, and insome embodiments, receivers 120 and 130 are rigidly affixed to thesurface. Computer 110 receives the time-of-arrival information fromreceivers 120 and 130, calculates the distance of ultrasonic pen 140from each receiver, and triangulates the position of ultrasonic pen 140versus time. Some embodiments include more than two receivers, thusproviding better accuracy through redundant information, or a largercoverage range.

[0018] In operation, a user creates a signature shape 160 on paper 150using ultrasonic pen 140. Receivers 120 and 130 track the location ofthe pen versus time, and computer 110 stores the signature shape as partof a signature to be authenticated. In various embodiments of thepresent invention, additional data are collected and included as part ofthe signature to be authenticated. For example, in some embodiments,computer 110 also calculates velocity and acceleration of the pen versustime, and includes these data as part of the signature to beauthenticated. Also for example, in some embodiments, ultrasonic pen 140includes a pressure sensitive tip that provides information representingpressure applied to the pen tip versus time, and this pressure data isincluded as part of the signature to be authenticated. Also for example,the location or identity of computer 110 can be included as part of thesignature to be authenticated. The location of computer 110 can berepresented by a phone number, and the identity of computer 110 can berepresented as an internet protocol (IP) address. A time stamprepresenting the time of the signature can also be included.

[0019] Computer 110 includes a library of signatures to which thecollected signature can be compared and possibly authenticated. Forexample, the signature of a person named “John Doe” can be stored in asignature library within computer 110. John Doe's signature data withinthe library can include any combination of signature data, including forexample, signature shape, pen location versus time, pen velocity versustime, pen acceleration versus time, pressure applied to the pen tipversus time, computer location or identity, and time stamp. Computer 110compares the signature to be authenticated to data in the signaturelibrary to determine if the signature is authentic.

[0020] The method and apparatus of the present invention allow a user towrite out a signature using a pen that produces ink on paper, therebyproviding a comfortable signature experience. Most people are accustomedto using an ink pen when providing a signature, and ultrasonic pen 140,while allowing the collection of data over and above signature shape,provides the “ink pen experience” that is comfortable for most people.Additionally, ultrasonic pen 140 provides signature information usingtransmitters, and so the pen does not need to be tethered. Again, mostpeople are more comfortable using a non-tethered pen, and in thisregard, ultrasonic pen 140 provides a signature experience that is closeto a natural ink pen experience.

[0021] Providing a signature experience that is close to a natural inkpen experience allows a user to provide a natural signature that is notbiased as a result of the tools used. For example, John Doe mightprovide a biased signature if forced to read his signature on a displaythat is not co-located with the pen tip. This is referred to as the“write here, look there” problem. Also for example, John Doe mightprovide a biased signature if forced to write on a surface other thanpaper. A system that forces John Doe to sign on a spongy pad may biashis signature, as might a system that forces John to sign inside a smallbox using a tethered pen. The method and apparatus of the presentinvention do not suffer from these potentially signature-biasingproblems, in part because a signature experience is provided that isclose to a natural ink pen signature experience.

[0022]FIG. 2 shows a signature collection system and remoteauthentication system. System 200 includes ultrasonic pen 140, receivers120 and 130, network interface 210, network 220, authentication computer230, and signature library 240. Some components of system 200 are commonto both system 200 and system 100 (FIG. 1). For example, ultrasonic pen140 and receivers 120 and 130 are common to both systems. In system 200,receivers 120 and 130 are coupled to network interface 210, which iscoupled to authentication computer 230 through network 220.

[0023] In some embodiments, network interface 210 is a dedicatedcomputer operative to collect the data representing the signature to beauthenticated. For example, network interface 210 collects the signatureshape and the pen location versus time, calculates the velocity andacceleration of the pen, and bundles this with information describingthe pressure applied to the pen tip versus time. In addition, networkinterface 210 can bundle all of this information with a time stamp and aphone number or IP address. Network interface 210 also includes antenna270 to receive a higher frequency signal from ultrasonic pen 140.

[0024] In operation, a user creates signature shape 160 in ink on paper150 using ultrasonic pen 140. Ultrasonic pen 140 periodically transmitspulses of ultrasonic energy and bursts of higher frequency energy.Antenna 270 receives the higher frequency energy from ultrasonic pen140. Network interface 210 includes a higher frequency receiver coupledto antenna 270, which provides a time base to network interface 210.Receivers 120 and 130 receive the ultrasonic bursts from ultrasonic pen140, and provide a time-of-arrival to network interface 210, which thentriangulates the position of ultrasonic pen 140.

[0025] Antenna 170 is shown affixed to receiver 120 in FIG. 1, andantenna 270 is shown affixed to network interface 210 in FIG. 2. Inembodiments represented by FIG. 1, a higher frequency receiver isco-located with ultrasonic receiver 120, and in embodiments representedby FIG. 2, a higher frequency receiver is co-located with networkinterface 210. These embodiments employ a higher frequency receiver,such as an RF receiver, that benefits from an antenna like the typeshown. In other embodiments, different antenna types are employed. Forexample, when either network interface 210 or ultrasonic receiver 120include an IR receiver, the antenna may include an IR detector and alens. In general, any suitable type of antenna or other energy receivingapparatus can be utilized to receive higher frequency energy fromultrasonic pen 140. Furthermore, the higher frequency receiver can beco-located with any other component in the system, and in someembodiments, is a stand-alone receiver.

[0026] Network interface 210 bundles the data in the signature to beauthenticated into a packet, and sends the packet through network 220 toauthentication computer 230. In some embodiments, network interface 210compresses and encrypts the signature prior to sending it throughnetwork 220. Network 220 is any type of network capable of transportingthe signature to be authenticated to authentication computer 230. Forexample, in some embodiments, network 220 is a dedicated network withinan enterprise such as a retail sales establishment, and in otherembodiments, network 220 is an open network such as the internet.

[0027] In some embodiments, network interface 210 includes a wirelessinterface to communicate with authentication computer 230. In theseembodiments, network 220 represents the air medium between networkinterface 210 and authentication computer 230. In operation, ultrasonicreceivers 120 and 130 receive ultrasonic bursts, and a higher frequencyreceiver (not shown) receives higher frequency bursts, all of which areprovided to network interface 210. Network interface 210 thencommunicates with authentication computer 230 through a wirelessinterface.

[0028] Authentication computer 230 receives, de-crypts and de-compressesthe signature to be authenticated, and compares it to signatures withinsignature library 240. In some embodiments, authentication computer 230adds the signature to signature library 240 when it cannot beauthenticated.

[0029] System 200 can be useful in applications where a completecomputer is not co-located with paper 150 and ultrasonic pen 140. Forexample, in point-of-sale (POS) applications, a customer may sign areceipt at a location where it is inconvenient to locate authenticationcomputer 230. In these applications, network interface 210 and receivers120 and 130 can be located on a surface accessible to a customer, andauthentication computer 230 can be under the surface, in a differentroom, or in a different building altogether.

[0030]FIG. 3 shows an ultrasonic pen. Ultrasonic pen 140 includes tip310, ultrasonic transmitter 330 near tip 310, ink cartridge 320, andantenna 340. In some embodiments, ultrasonic pen is turned on whenpressure, shown as force vector 360, is applied to ultrasonic pen 140.When ultrasonic pen 140 is on, ultrasonic transmitter 330 emits pulsesof ultrasonic energy, and antenna 340 emits higher frequency bursts.Ultrasonic transmitter 330 can emit ultrasonic energy at any suitablefrequency. In some embodiments, ultrasonic transmitter 330 emits energyat between 40 kHz and 80 kHz. Ultrasonic pen 140 can be in any positionrelative to the receiving stations when the pen is turned on, and theposition of ultrasonic pen 140 can be triangulated by the receivingstations, or by computer 110 (FIG. 1).

[0031] Ultrasonic transmitter 330 is positioned near tip 310 which iscoupled to a pressure activated switch (not shown). When tip 310 ispressed against a surface such as paper 150 (FIG. 1), ultrasonictransmitter 330 turns on and emits ultrasonic energy. In someembodiments, ultrasonic transmitter 330 is a cylindrical,omni-directional transmitter made of a polymer material. The cylindricalshape allows tip 310 to pass through, and the omni-directional patternallows the ultrasonic receivers to receive the ultrasonic energy at anacceptable amplitude. In some embodiments, ultrasonic transmitter 330includes a plurality of piezoelectric panels arranged in a cylindricalpattern. In other embodiments, ultrasonic transmitter 330 includes asingle cylindrical piezoelectric transmitter.

[0032] Antenna 340 is coupled to a higher frequency transmitter (notshown) within ultrasonic pen 140. Antenna 340 is an omni-directionalantenna configured to transmit bursts of higher frequency energy whenultrasonic pen 140 is turned on. In some embodiments, antenna 340 ispositioned near the center of ultrasonic pen 140 as shown in FIG. 3, andin other embodiments, antenna 340 is near tip 310. In still otherembodiments, antenna 340 is near end 350. Antenna 340 is made ofmaterial suitable for the higher frequency at which it operates. Forexample, in embodiments that include an RF transmitter, antenna 340 canbe made of a metallic material suitable for radiating RF energy. Inembodiments that include an IR transmitter, antenna 340 can be made oflight emitting diodes.

[0033] Pen tip 310 dispenses ink from ink cartridge 320 when tip 310 isapplied to paper. In addition, pen tip 310 is coupled to a pressuresensitive switch to turn on ultrasonic pen 140 as previously described.In some embodiments, the pressure sensitive switch is also a pressuresensor that senses the amount of pressure applied to pen tip 310. Thepressure information that is generated by the pressure sensitive switchis transmitted at the higher frequency by antenna 340. This pressureinformation is received by the higher frequency receiver (FIG. 1), andincluded as part of the signature to be authenticated.

[0034]FIG. 4 shows a block diagram of an ultrasonic pen. Ultrasonic pen400 represents embodiments of ultrasonic pen 140 (FIGS. 1-3) thatinclude a radio frequency (RF) transmitter. Ultrasonic pen 400 includespressure sensor 410, analog-to-digital converter (A/D) 420, modulator430, RF transmitter 450, and ultrasonic transmitter 440. Ultrasonictransmitter 440 and RF transmitter 450 are turned on by pressure sensor410. This occurs when pressure is applied to the tip of ultrasonic pen400. When turned on, RF transmitter 440 transmits periodic RF burstsfrom ultrasonic pen 400.

[0035] Pressure sensor 410 supplies A/D 420 with a signal that includesinformation describing the pressure applied to the tip of the pen. Forexample, a signal having an amplitude that is proportional to pressurecan be provided on node 412. A/D 420 digitizes this information andprovides it to modulator 430. Modulator 430 receives the digitalpressure information from A/D 420 and drives RF transmitter 450. In someembodiments, each RF burst transmitted by RF transmitter 450 includespressure information.

[0036] Modulator 430 can be any type of modulator suitable formodulating pressure information on an RF burst. In some embodiments,modulator 430 is an amplitude modulator, and in other embodiments,modulator 430 is a phase modulator. In still other embodiments,modulator 430 is a modulator that combines amplitude and phasemodulation.

[0037] The frequency of the RF bursts determines how often pressureinformation is transmitted by ultrasonic pen 400. For example, insystems that transmit 100 bursts per second, pressure information isalso transmitted 100 times per second. In some embodiments, the samplingrate of pressure information is equal to the burst frequency of thetransmitter, and in other embodiments, the sampling rate of pressureinformation is greater than the burst frequency of the transmitter. Inembodiments that have unequal sampling rates and burst frequencies, eachburst includes more than one discrete pressure data point. For example,if the burst frequency is 100 times per second, and the pressuresampling rate is 200 samples per second, each RF burst can include twopressure data points.

[0038] RF transmitter 450 is an example of a higher frequencytransmitter suitable for use as a time reference between the pen and thereceiving station. Other types of higher frequency transmitters can beused without departing from the scope of the present invention. Forexample, in other embodiments, an infrared (IR) transmitter is used.

[0039]FIG. 5 shows a block diagram of an ultrasonic pen with an IRtransmitter. Ultrasonic pen 500, like ultrasonic pen 400 (FIG. 4),represents various embodiments of ultrasonic pen 140 (FIGS. 1-3). Unlikeultrasonic pen 400, ultrasonic pen 500 includes infrared transmitter550. Ultrasonic pen 500 also includes pressure sensor 510, modulator530, and ultrasonic transmitter 540.

[0040] In operation, pressure sensor 510 turns on ultrasonic transmitter540 and IR transmitter 550 when pressure is applied to the tip of thepen. Ultrasonic transmitter 540 transmits ultrasonic pulses so that theposition of ultrasonic pen 500 can be triangulated. IR transmitter 550transmits infrared bursts that include pressure information provided bymodulator 530. Modulator 530 is an analog modulator that modulates usinga variable voltage rather than a digital signal.

[0041] Ultrasonic pen 500 is shown with an IR transmitter and without anA/D converter. Ultrasonic pen 400 (FIG. 4), on the other hand, is shownwith an RF transmitter and also with an A/D. Some embodiments include anRF transmitter without an A/D, and other embodiments include an IRtransmitter with an AID. One skilled in the art will understand that thevarious features shown can be combined in different combinations toarrive at different embodiments of the present invention.

[0042]FIG. 6 shows a block diagram of receivers for use in conjunctionwith a signature collection system. Receiver 600 include ultrasonicreceivers 640, 642, and RF receiver 644, corresponding to the ultrasonicreceivers and higher frequency receiver described with reference toFIGS. 1 and 2. Ultrasonic receiver 640 includes ultrasonic sensor 602,amplifier 606, analog-to-digital converter (A/D) 610, and latch 614.Ultrasonic receiver 642 includes ultrasonic sensor 604, amplifier 608,A/D 612, and latch 616. RF receiver 644 includes antenna 618, filter620, demodulator 626, and detector 622. Each of receivers 640, 642, and644 provides information to wireless interface 624.

[0043] RF receiver 644 receives radio frequency bursts at antenna 618,detects the time-of-arrival of the bursts at detector 622, and providesthe time-of-arrival to wireless interface 624. In addition, demodulator626 demodulates data present in the bursts, and provides the data towireless interface 624. In some embodiments, the demodulated datarepresents pressure applied to a pen tip. In some embodiments, digitalinformation is demodulated by demodulator 626. In these embodiments, anynumber of symbols can be demodulated per RF burst, and any number ofbits can be included in each symbol. In other embodiments, analoginformation is demodulated by demodulator 626. In some of theseembodiments, wireless interface 624 includes an analog-to-digitalconverter to digitize the demodulated data.

[0044] Antenna 618 corresponds to antennas 170 (FIG. 1) and 270 (FIG.2). Accordingly, RF receiver 644 can be co-located with either ofultrasonic receivers 640 and 642, or can be co-located with wirelessinterface 424. Embodiments represented by FIG. 6 include an RF receiveras a higher frequency receiver. Other embodiments include other types ofhigher frequency receivers. For example, some embodiments include an IRreceiver rather than an RF receiver.

[0045] Ultrasonic receivers 640 and 642 receive ultrasonic pulses, andprovide the time-of-arrival of the ultrasonic pulses to wirelessinterface 624. Wireless interface 624 provides the time-of-arrivalinformation and the demodulated data to an authentication computer suchas computer 110 (FIG. 1) or computer 230 (FIG. 2). In some embodiments,wireless interface 624 includes a processor that determines thedifference between the times-of-arrival of RF bursts and ultrasonicpulses, and provides the difference to the computer. In someembodiments, wireless interface 624 uses an industry standard interfacesuch as the “Bluetooth” standard. In other embodiments, wirelessinterface 624 uses a dedicated interface between the authenticationcomputer and wireless interface 624.

[0046]FIG. 7 shows a flowchart of a method for collecting signatures tobe authenticated. Method 700 begins at block 710 where an internetprotocol (IP) address or a telephone number is collected. The IP addressor telephone number represents an identity of the location from whichthe signature is retrieved. For example, referring now back to FIG. 2,network interface 210 may have an IP address when network 220 is theinternet, or network interface 210 may have a phone number associatedtherewith when network 220 is a telephone network. The IP address ortelephone number can be used by an authentication computer to verify thelocation from which the signature is retrieved. In some embodiments, asignature is authentic only if it is retrieved from a particularlocation, and in other embodiments, a signature is authentic only if itis retrieved from one of a list of locations. In still otherembodiments, the location information is not utilized forauthentication, but rather is used to document the location for archivalpurposes.

[0047] Block 720 involves the collection of signature stroke coordinatesof a pen versus time. This includes receiving from the pen higherfrequency bursts at a higher frequency receiver, and receiving from thepen ultrasonic pulses at more than one ultrasonic receiver. In someembodiments, the higher frequency bursts travel from the pen to thereceiver at approximately the speed of light, and the ultrasonic pulsestravel from the pen to the receiver at approximately the speed of sound.These two speeds differ by many orders of magnitude, so the higherfrequency bursts can be used as a time reference to calculate thetime-of-arrival of the ultrasonic pulses at the more than one receiver.The times-of-arrival are then used to triangulate the position of thepen relative to the receivers. This represents the signature stokecoordinates versus time.

[0048] In block 730, pressure information versus time is collected. Aspreviously described, in some embodiments, the higher frequency burstsinclude information describing the amount of pressure applied to the pentip. This information is demodulated and digitized to generate thepressure applied to the pen versus time.

[0049] In block 740, the stroke coordinates versus time (from block 720)are used to construct the ink shape made by the pen. In blocks 750 and760, the velocity of the pen and the acceleration of the pen arecalculated using the stroke coordinates versus time.

[0050] In block 770, a time stamp is added. The time stamp representsthe time that the signature was made. Like the location informationdescribed above, the time stamp is used for authentication in someembodiments, and in other embodiments, the time stamp is used todocument the time of the signature for archival purposes.

[0051] The signature information collected in method 700 is bundled aspart of a signature to be authenticated. In some embodiments, thesignature is authenticated by a computer that is remote relative to theapparatus that performs the collection. In these embodiments, thesignature is prepared for authentication in various ways. In block 780,the signature is prepared by encrypting the collected data. In someembodiments, the encrypted signature data is packetized into a packetand sent across a network to an authentication computer. This is usefulin embodiments that use a public network for network 220 (FIG. 2). Forexample, when the signature is to be transmitted over the internet,block 780 can provide encryption services to reduce the chance that anunwanted recipient will gain access to the signature data.

[0052] Method 700 can be performed by a general purpose computer such ascomputer 110 (FIG. 1), or by a specialized computer designedspecifically for signature collection, such as network interface 210(FIG. 2). The computer instructions useful to perform the method can bestored on any type of suitable media, including random access memory(RAM), read-only memory (ROM), CDROM, or floppy or hard disks. Forexample, computer 110 (FIG. 1) includes disk drive 180. Disk drive 180is one example of an article having a machine readable medium suitableto store computer instructions for method 700.

[0053] While the blocks within method 700 are presented in FIG. 7 in aspecific order, this is not meant to imply that the actions need beperformed in the order shown. For example, collecting an IP address ortelephone number can be performed at any point within method 700.Similarly, any block shown can be performed in an order different fromthat presented in the figure without departing from the scope of thepresent invention.

[0054] It is to be understood that the above description is intended tobe illustrative, and not restrictive. Many other embodiments will beapparent to those of skill in the art upon reading and understanding theabove description. The scope of the invention should, therefore, bedetermined with reference to the appended claims, along with the fullscope of equivalents to which such claims are entitled.

What is claimed is:
 1. An ultrasonic pen comprising: a pressuresensitive tip to generate a pressure signal; an ultrasonic transmitterto transmit pulses for position determination; and a second transmitterto transmit the pressure signal.
 2. The ultrasonic pen of claim 1wherein the pressure sensitive tip includes an ink dispenser.
 3. Theultrasonic pen of claim 1 wherein the ultrasonic transmitter and thesecond transmitter emit energy at different frequencies.
 4. Theultrasonic pen of claim 3 wherein the ultrasonic transmitter emitsultrasonic energy between substantially forty kilohertz and eightykilohertz.
 5. The ultrasonic pen of claim 1 further comprising apressure activated switch coupled to a first end of the pen, wherein thepressure activated switch is operable to turn on the ultrasonictransmitter when pressure is applied to the first end of the pen.
 6. Theultrasonic pen of claim 5 wherein the ultrasonic transmitter comprises apiezoelectric material.
 7. The ultrasonic pen of claim 6 wherein thepiezoelectric material is arrange in a cylinder.
 8. The ultrasonic penof claim 1 wherein the second transmitter comprises an infraredtransmitter.
 9. The ultrasonic pen of claim 8 wherein the secondtransmitter is configured to provide a time reference signal.
 10. Theultrasonic pen of claim 1 wherein the second transmitter comprises aradio frequency transmitter.
 11. The ultrasonic pen of claim 1 whereinthe ultrasonic transmitter and the second transmitter are bursttransmitters.
 12. A digital signature collection system comprising: afirst ultrasonic receiver; a second ultrasonic receiver mountable on asurface a distance from the first ultrasonic receiver; a higherfrequency receiver; and an ultrasonic pen having an pressure sensitiveink cartridge to dispense ink and to generate a pressure signal, theultrasonic pen being configured to transmit the pressure signal to thehigher frequency receiver, and to transmit ultrasonic energy to thefirst and second ultrasonic receivers.
 13. The digital signaturecollection system of claim 12 further comprising a computer coupled tothe first and second ultrasonic receivers and the higher frequencyreceiver.
 14. The digital signature collection system of claim 13wherein the computer is configured to receive time-of-arrival data fromthe first and second ultrasonic receivers, and configured to receivepressure signal information from the higher frequency receiver.
 15. Thedigital signature collection system of claim 14 wherein the computer isfurther configured to determine a location of the ultrasonic pen versustime, a velocity of the ultrasonic pen versus time, and an accelerationof the ultrasonic pen versus time.
 16. The digital signature collectionsystem of claim 12 wherein the higher frequency receiver comprises aninfrared receiver.
 17. The digital signature collection system of claim12 wherein the higher frequency receiver comprises a radio frequencyreceiver.
 18. A method of operating a digital signature collectionsystem comprising: receiving ultrasonic energy at a plurality ofultrasonic receivers; receiving pressure information at a higherfrequency receiver; determining a location of an ultrasonic transmitterversus time, from the ultrasonic energy to determine a signature shape;determining an applied pressure versus time, from the pressureinformation; and preparing the signature shape and the applied pressurefor comparison with a reference.
 19. The method of claim 18 whereinpreparing for comparison comprises encrypting.
 20. The method of claim18 further comprising determining velocity of the ultrasonic transmitterversus time, from the ultrasonic energy.
 21. The method of claim 18further comprising determining acceleration of the ultrasonictransmitter versus time, from the ultrasonic energy.
 22. The method ofclaim 21 wherein preparing comprises: encrypting the signature shape,the applied pressure, and the acceleration into a packet; and sendingthe packet to a processor for comparison with a reference signature thatincludes signature shape, applied pressure, and acceleration.
 23. Themethod of claim 22 wherein preparing further comprises combining an IPaddress in the packet.
 24. The method of claim 22 wherein preparingfurther comprises combining a phone number in the packet.
 25. An articlehaving a machine-readable medium, the machine-readable medium havinginstructions stored thereon for a method comprising: determining thelocation of an ultrasonic transmitter versus time, from informationreceived from a plurality of ultrasonic receivers; and determiningapplied pressure versus time, of pressure applied to the ultrasonictransmitter from information received by a higher frequency transmitter.26. The article of claim 25 wherein the method further comprises:determining velocity of the ultrasonic transmitter versus time, from theinformation received from the plurality of ultrasonic receivers.
 27. Thearticle of claim 25 wherein the method further comprises: encrypting thelocation, the pressure, and the velocity into a packet; and sending thepacket to a processor for comparison with a reference that includeslocation, pressure, and velocity.
 28. The article of claim 27 whereinthe method further comprises encrypting location information into thepacket.
 29. The article of claim 27 wherein the method further comprisescomparing the packet with the reference.